1. Field of the Invention
The invention relates to access systems and particularly to DSL (Digital Subscriber Line) access systems.
2. Description of the Related Art
DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to e.g. homes and small businesses over ordinary copper telephone lines. Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to a subscriber directly as digital data and this allows a much wider bandwidth to be used for transmitting the data than in traditional telephone systems. Several modulation technologies are used by various kinds of DSL, although these are being standardized by the International Telecommunication Union (ITU).
DSL allows a signal to be separated so that some of the bandwidth is used to transmit an analog signal whereby it is possible to use e.g. a telephone and a computer on the same line and at the same time.
Voice is being introduced into the DSL access systems (VoDSL=Voice over DSL). For example, 32×64 kbps voice channels may be multiplexed among other data traffic into one DSL line, as depicted in FIG. 1. The voice channels may additionally be compressed in a customer premises multiplexing device (which includes a DSL modem). Voice compression extends the multiplexing capacity e.g. by a factor of 8:1 (with a G.729 voice codec). The encoded voice packets are then transferred in ATM (Asynchronous Transfer Mode) cells over DSL, or in IP (Internet Protocol) packets (over ATM) over DSL via the DSL Access Multiplexer (DSLAM), to the voice gateway which decodes the voice coding and interfaces the telephone switch with a 64 kbps G.711 format.
If a voice call takes place from a DSL access system to an other system employing a voice coding method, e.g. another DSL access system or a mobile phone system (the operation of a mobile communication network involves a transcoder on the connection between a mobile station and a mobile services switching center), voice encoding and decoding are performed twice for the call. This is known as tandem coding. Tandem coding presents a problem as it degrades speech quality due to the extra voice encoding and decoding.